The "apocalypse" part.
Source.Which is weird, right? Seems like it'd be hard to miss the end of the whole world.
We don't love zombies because of the monster: We love zombies because of the unique scenario the monster creates. Zombies are just slower horror versions of the bandits from The Road Warrior, or stupider versions of the vampires from I Am Legend. (The book. We will not now and shall never discuss the Will Smith "movie" version.) And yet every zombie game takes place with you either trying to prevent or fighting your way through the zombie apocalypse. They should be taking place, say, a year after it. A year after the large-scale fighting has died out; after the survivors have been whittled down; after what few people are left have already executed their survival plans and are now just dealing with the fallout in a world of the undead.
My perfect zombie game would be an open world one, in the style of Grand Theft Auto IV or the Saints Row series, but with an emphasis on completeness instead of size. See, I think the ambitious scale of open world games these days is actually a weakness. They create vast play areas, sure, but they're ultimately empty and inaccessible. I know that's a matter of hardware and design limitations -- you can't satisfactorily model the inside of every building in LA with current hardware and realistic resources -- but why hasn't anybody scaled down? Instead of creating a two-dimensional theater backdrop of New York City, why not give us every living, breathing inch of Small Town America? A little backwoods city with a modest downtown area, surrounded by a bit of suburb and wilderness, and every inch of it totally realized. You can't tell me we can build World of Warcraft but we can't nail down a decent facsimile of Redmond, Oregon. Give me a small town in the country, with every room in every building built out, as much of it interactive as possible, and the whole damn thing just friggin' lousy with the undead.
Source.Like, objectively just too many undead. So many they don't even have stuff to do anymore; they just loiter.